In a conventional switching regulator, such as a 6-pin package boost converter shown in FIG. 1, an error amplifier U1 rapidly charges the compensation capacitor C1 during the startup. Hence, the circuit charges the output capacitor Cout with its maximum duty cycle. This may cause a surge that overloads the input supply or cause over-voltage at the output. The surge can be avoided by using a soft-start controller to slowly ramp up the voltage at C1 and limit the energy being transferred to Cout.
FIG. 2 shows a prior art soft-start method. During the power-off period, capacitors C1 and C2 are discharged to ground GND. When the regulator starts up, C2 is slowly charged by a small current from a transistor MP3. The output of the error amplifier U1 is clamped by a transistor Q1 whose base terminal is coupled to C2 and a PIN SS. Q1 regulates U1's output voltage to rise at the same rate as the voltages at C2 and SS. This method allows the circuit to slowly start up the output voltage with a minimum duty cycle and avoids the over-voltage surge problem. However, it requires an extra capacitor and an extra pin which could be a problem if the number of pins is limited and/or the capacitor is too large to be integrated into a single integrated circuit package.
FIG. 3 shows another prior art soft-start method, which does not require the SS pin and capacitor C2 of the circuit of FIG. 2. During the power off period, a capacitor C1 is discharged to ground through a transistor MN3. When the system starts up, the output terminal of the error amplifier ERROR AMP U1 is disconnected from the compensation network comprising a resistor R1 and a compensation capacitor C1. The pin COMP is connected to a small pull-up current source through a transistor MP4, which slowly charges the compensation capacitor C1 so that the voltage at COMP rises slowly to limit the energy being delivered to the output. When the voltage at the pin FB equals to a reference voltage at a comparator U3, the output of U3 sets the flip-flop RSFF U4 so that COMP is disconnected from the pull up current through MP4 while ERROR AMP U1 is connected to COMP through activated transistors, MN5 and MP5, for a normal operation. A disadvantage with this method is that the output of ERROR AMP U1 is at the high rail while it remains disconnected from COMP during the start-up. When the output of ERROR AMP U1 is connected to COMP at the end of soft start cycle, it may cause the voltage at COMP to jump much higher and results in a possible voltage overshot at the output.